FINFET including source and drain regions with dopant diffusion blocking superlattice layers to reduce contact resistance

ABSTRACT

A FINFET may include a semiconductor fin, spaced apart source and drain regions in the semiconductor fin with a channel region extending therebetween, and at least one dopant diffusion blocking superlattice dividing at least one of the source and drain regions into a lower region and an upper region with the upper region having a same conductivity and higher dopant concentration than the lower region. The dopant diffusion blocking superlattice may include a plurality of stacked groups of layers, with each group of layers comprising a plurality of stacked base semiconductor monolayers defining a base semiconductor portion, and at least one non-semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor portions. The semiconductor device may further include a gate on the channel region.

TECHNICAL FIELD

The present disclosure generally relates to semiconductor devices and,more particularly, to semiconductor devices with enhanced contactconfigurations and related methods.

BACKGROUND

Structures and techniques have been proposed to enhance the performanceof semiconductor devices, such as by enhancing the mobility of thecharge carriers. For example, U.S. Patent Application No. 2003/0057416to Currie et al. discloses strained material layers of silicon,silicon-germanium, and relaxed silicon and also including impurity-freezones that would otherwise cause performance degradation. The resultingbiaxial strain in the upper silicon layer alters the carrier mobilitiesenabling higher speed and/or lower power devices. Published U.S. PatentApplication No. 2003/0034529 to Fitzgerald et al. discloses a CMOSinverter also based upon similar strained silicon technology.

U.S. Pat. No. 6,472,685 B2 to Takagi discloses a semiconductor deviceincluding a silicon and carbon layer sandwiched between silicon layersso that the conduction band and valence band of the second silicon layerreceive a tensile strain. Electrons having a smaller effective mass, andwhich have been induced by an electric field applied to the gateelectrode, are confined in the second silicon layer, thus, an re-channelMOSFET is asserted to have a higher mobility.

U.S. Pat. No. 4,937,204 to Ishibashi et al. discloses a superlattice inwhich a plurality of layers, less than eight monolayers, and containinga fractional or binary or a binary compound semiconductor layer, arealternately and epitaxially grown. The direction of main current flow isperpendicular to the layers of the superlattice.

U.S. Pat. No. 5,357,119 to Wang et al. discloses a Si—Ge short periodsuperlattice with higher mobility achieved by reducing alloy scatteringin the superlattice. Along these lines, U.S. Pat. No. 5,683,934 toCandelaria discloses an enhanced mobility MOSFET including a channellayer comprising an alloy of silicon and a second materialsubstitutionally present in the silicon lattice at a percentage thatplaces the channel layer under tensile stress.

U.S. Pat. No. 5,216,262 to Tsu discloses a quantum well structurecomprising two barrier regions and a thin epitaxially grownsemiconductor layer sandwiched between the barriers. Each barrier regionconsists of alternate layers of SiO2/Si with a thickness generally in arange of two to six monolayers. A much thicker section of silicon issandwiched between the barriers.

An article entitled “Phenomena in silicon nanostructure devices” also toTsu and published online Sep. 6, 2000 by Applied Physics and MaterialsScience & Processing, pp. 391-402 discloses a semiconductor-atomicsuperlattice (SAS) of silicon and oxygen. The Si/O superlattice isdisclosed as useful in a silicon quantum and light-emitting devices. Inparticular, a green electroluminescence diode structure was constructedand tested. Current flow in the diode structure is vertical, that is,perpendicular to the layers of the SAS. The disclosed SAS may includesemiconductor layers separated by adsorbed species such as oxygen atoms,and CO molecules. The silicon growth beyond the adsorbed monolayer ofoxygen is described as epitaxial with a fairly low defect density. OneSAS structure included a 1.1 nm thick silicon portion that is abouteight atomic layers of silicon, and another structure had twice thisthickness of silicon. An article to Luo et al. entitled “Chemical Designof Direct-Gap Light-Emitting Silicon” published in Physical ReviewLetters, Vol. 89, No. 7 (Aug. 12, 2002) further discusses the lightemitting SAS structures of Tsu.

U.S. Pat. No. 7,105,895 to Wang et al. discloses a barrier buildingblock of thin silicon and oxygen, carbon, nitrogen, phosphorous,antimony, arsenic or hydrogen to thereby reduce current flowingvertically through the lattice more than four orders of magnitude. Theinsulating layer/barrier layer allows for low defect epitaxial siliconto be deposited next to the insulating layer.

Published Great Britain Patent Application 2,347,520 to Mears et al.discloses that principles of Aperiodic Photonic Band-Gap (APBG)structures may be adapted for electronic bandgap engineering. Inparticular, the application discloses that material parameters, forexample, the location of band minima, effective mass, etc., can betailored to yield new aperiodic materials with desirable band-structurecharacteristics. Other parameters, such as electrical conductivity,thermal conductivity and dielectric permittivity or magneticpermeability are disclosed as also possible to be designed into thematerial.

Furthermore, U.S. Pat. No. 6,376,337 to Wang et al. discloses a methodfor producing an insulating or barrier layer for semiconductor deviceswhich includes depositing a layer of silicon and at least one additionalelement on the silicon substrate whereby the deposited layer issubstantially free of defects such that epitaxial silicon substantiallyfree of defects can be deposited on the deposited layer. Alternatively,a monolayer of one or more elements, preferably comprising oxygen, isabsorbed on a silicon substrate. A plurality of insulating layerssandwiched between epitaxial silicon forms a barrier composite.

Despite the existence of such approaches, further enhancements may bedesirable for using advanced semiconductor materials and processingtechniques to achieve improved performance in semiconductor devices.

SUMMARY

A FINFET may include a semiconductor fin, spaced apart source and drainregions in the semiconductor fin with a channel region extendingtherebetween, and at least one dopant diffusion blocking superlatticedividing at least one of the source and drain regions into a lowerregion and an upper region with the upper region having a sameconductivity and higher dopant concentration than the lower region. Thedopant diffusion blocking superlattice may include a plurality ofstacked groups of layers, with each group of layers comprising aplurality of stacked base semiconductor monolayers defining a basesemiconductor portion, and at least one non-semiconductor monolayerconstrained within a crystal lattice of adjacent base semiconductorportions. The semiconductor device may further include a gate on thechannel region.

More particularly, the at least one dopant diffusion blockingsuperlattice may comprise a respective dopant diffusion blockingsuperlattice for each of the source and drain regions. In an exampleembodiment, the upper region may be raised above an upper surface of thesemiconductor layer. Furthermore, the lower region may comprise adifferent material than the upper region. By way of example, the lowerregion may comprise silicon, and the upper region may comprise silicongermanium. In accordance with another example, the lower region maycomprise silicon germanium, and the upper region may comprise silicon.

The FINFET may further include a metal contact on the upper region. Byway of example, the metal contact may comprise at least one of atitanium, cobalt, nickel and platinum. In addition, the FINFET may alsoinclude a semiconductor substrate from which the semiconductor finextends vertically upward, and an insulator on the semiconductorsubstrate and on opposite sides of the semiconductor fin. Also by way ofexample, the base semiconductor monolayers may comprise silicon, and theat least one non-semiconductor monolayer may comprise oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a greatly enlarged schematic cross-sectional view of asuperlattice for use in a semiconductor device in accordance with anexample embodiment.

FIG. 2 is a perspective schematic atomic diagram of a portion of thesuperlattice shown in FIG. 1.

FIG. 3 is a greatly enlarged schematic cross-sectional view of anotherembodiment of a superlattice in accordance with an example embodiment.

FIG. 4A is a graph of the calculated band structure from the gamma point(G) for both bulk silicon as in the prior art, and for the 4/1 Si/Osuperlattice as shown in FIGS. 1-2.

FIG. 4B is a graph of the calculated band structure from the Z point forboth bulk silicon as in the prior art, and for the 4/1 Si/O superlatticeas shown in FIGS. 1-2.

FIG. 4C is a graph of the calculated band structure from both the gammaand Z points for both bulk silicon as in the prior art, and for the5/1/3/1 Si/O superlattice as shown in FIG. 3.

FIG. 5 is schematic cross-sectional diagram of a semiconductor deviceincluding a dopant diffusion blocking superlattice dividing the sourceand drain regions to provide reduced source and drain contactresistance.

FIG. 6 is schematic cross-sectional diagram of a semiconductor deviceincluding source and drain regions divided by respective dopantdiffusion blocking superlattices and providing reduced source and draincontact resistance.

FIGS. 7A-7C are a series of schematic cross-sectional diagramsillustrating a method of making a semiconductor device with interimsource and drain superlattice layers to provide Schottky barrier heightmodulation by controlling interfacial insulator formation.

FIGS. 8-9 are a schematic cross-sectional diagrams of examplesemiconductor devices including source and drain regions divided byrespective superlattices and having a different upper and lowersemiconductor source/drain materials and metal contact regions.

FIG. 10 is a perspective view of a FINFET including source and drainregions divided by respective superlattices and providing reduced sourceand drain contact resistance.

FIG. 11 is cross-sectional view of the FINFET of FIG. 10 taken alongline A-A.

FIG. 12 is a cross-sectional view of an alternative embodiment of theFINFET of FIG. 10 taken along line A-A formed using a similar process tothat shown in FIGS. 7A-70.

FIGS. 13 and 14 are cross-sectional views of alternative embodiments ofthe FINFET of FIG. 10 taken along line A-A which includes similar sourceand drain configurations to the devices of FIGS. 8 and 9, respectively.

FIG. 15 is a series of schematic atomic level diagrams illustratingapproaches by which non-semiconductor insertion layers from the MSTfilms shown in FIGS. 1-4C may be used to provide the metal-semiconductorcontacts with proximate dopants for reduced Schottky barrier heights inthe embodiments of FIGS. 5-14.

FIG. 16 is a schematic cross-sectional diagram of a semiconductor deviceincluding a body contact with a dopant diffusion blocking superlatticeto provide reduced contact resistance in accordance with an exampleembodiment.

FIG. 17 is a schematic cross-sectional diagram of a semiconductor deviceincluding a back side body contact with a dopant diffusion blockingsuperlattice to provide reduced contact resistance in accordance with anexample embodiment.

FIG. 18 is a cross sectional view of an alternative contact structurewhich may be used to provide reduced contact resistance in accordancewith an example embodiment.

DETAILED DESCRIPTION

Example embodiments will now be described more fully hereinafter withreference to the accompanying drawings, in which the example embodimentsare shown. The embodiments may, however, be implemented in manydifferent forms and should not be construed as limited to the specificexamples set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete. Like numbers referto like elements throughout, and prime and multiple prime notation areused to indicate similar elements in different embodiments.

Generally speaking, the present disclosure relates to utilizing enhancedsuperlattice materials within source and drain regions to reduceSchottky barrier height and thereby decrease source and drain contactresistance. The enhanced semiconductor superlattice is also referred toas an “MST” layer or “MST technology” in this disclosure and theaccompanying drawings.

More particularly, the MST technology relates to advanced semiconductormaterials such as the superlattice 25 described further below. Applicanttheorizes, without wishing to be bound thereto, that certainsuperlattices as described herein reduce the effective mass of chargecarriers and that this thereby leads to higher charge carrier mobility.Effective mass is described with various definitions in the literature.As a measure of the improvement in effective mass Applicants use a“conductivity reciprocal effective mass tensor”, M_(e) ⁻¹ and M_(h) ⁻¹for electrons and holes respectively, defined as:

${M_{e,{ij}}^{- 1}\left( {E_{F},T} \right)} = \frac{\sum\limits_{E > E_{F}}{\int_{B.Z.}{\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{i}\ \left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{j}\frac{\partial{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}}{\partial E}d^{3}k}}}{\sum\limits_{E > E_{F}}{\int_{B.Z.}{{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}d^{3}k}}}$for electrons and:

${M_{h,{ij}}^{- 1}\left( {E_{F},T} \right)} = \frac{- {\sum\limits_{E < E_{F}}{\int_{B.Z.}{\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{i}\ \left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{j}\frac{\partial{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}}{\partial E}d^{3}k}}}}{\sum\limits_{E < E_{F}}{\int_{B.Z.}{\left( {1 - {f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}} \right)d^{3}k}}}$

for holes, where f is the Fermi-Dirac distribution, E_(F) is the Fermienergy, T is the temperature, E(k,n) is the energy of an electron in thestate corresponding to wave vector k and the n^(th) energy band, theindices i and j refer to Cartesian coordinates x, y and z, the integralsare taken over the Brillouin zone (B.Z.), and the summations are takenover bands with energies above and below the Fermi energy for electronsand holes respectively.

Applicant's definition of the conductivity reciprocal effective masstensor is such that a tensorial component of the conductivity of thematerial is greater for greater values of the corresponding component ofthe conductivity reciprocal effective mass tensor. Again Applicanttheorizes without wishing to be bound thereto that the superlatticesdescribed herein set the values of the conductivity reciprocal effectivemass tensor so as to enhance the conductive properties of the material,such as typically for a preferred direction of charge carrier transport.The inverse of the appropriate tensor element is referred to as theconductivity effective mass. In other words, to characterizesemiconductor material structures, the conductivity effective mass forelectrons/holes as described above and calculated in the direction ofintended carrier transport is used to distinguish improved materials.

Applicant has identified improved materials or structures for use insemiconductor devices. More specifically, Applicant has identifiedmaterials or structures having energy band structures for which theappropriate conductivity effective masses for electrons and/or holes aresubstantially less than the corresponding values for silicon. Inaddition to the enhanced mobility characteristics of these structures,they may also be formed or used in such a manner that they providepiezoelectric, pyroelectric, and/or ferroelectric properties that areadvantageous for use in a variety of different types of devices, as willbe discussed further below.

Referring now to FIGS. 1 and 2, the materials or structures are in theform of a superlattice 25 whose structure is controlled at the atomic ormolecular level and may be formed using known techniques of atomic ormolecular layer deposition. The superlattice 25 includes a plurality oflayer groups 45 a-45 n arranged in stacked relation, as perhaps bestunderstood with specific reference to the schematic cross-sectional viewof FIG. 1.

Each group of layers 45 a-45 n of the superlattice 25 illustrativelyincludes a plurality of stacked base semiconductor monolayers 46defining a respective base semiconductor portion 46 a-46 n and an energyband-modifying layer 50 thereon. The energy band-modifying layers 50 areindicated by stippling in FIG. 1 for clarity of illustration.

The energy band-modifying layer 50 illustratively includes onenon-semiconductor monolayer constrained within a crystal lattice ofadjacent base semiconductor portions. By “constrained within a crystallattice of adjacent base semiconductor portions” it is meant that atleast some semiconductor atoms from opposing base semiconductor portions46 a-46 n are chemically bound together through the non-semiconductormonolayer 50 therebetween, as seen in FIG. 2. Generally speaking, thisconfiguration is made possible by controlling the amount ofnon-semiconductor material that is deposited on semiconductor portions46 a-46 n through atomic layer deposition techniques so that not all(i.e., less than full or 100% coverage) of the available semiconductorbonding sites are populated with bonds to non-semiconductor atoms, aswill be discussed further below. Thus, as further monolayers 46 ofsemiconductor material are deposited on or over a non-semiconductormonolayer 50, the newly deposited semiconductor atoms will populate theremaining vacant bonding sites of the semiconductor atoms below thenon-semiconductor monolayer.

In other embodiments, more than one such non-semiconductor monolayer maybe possible. It should be noted that reference herein to anon-semiconductor or semiconductor monolayer means that the materialused for the monolayer would be a non-semiconductor or semiconductor ifformed in bulk. That is, a single monolayer of a material, such assilicon, may not necessarily exhibit the same properties that it wouldif formed in bulk or in a relatively thick layer, as will be appreciatedby those skilled in the art.

Applicant theorizes without wishing to be bound thereto that energyband-modifying layers 50 and adjacent base semiconductor portions 46a-46 n cause the superlattice 25 to have a lower appropriateconductivity effective mass for the charge carriers in the parallellayer direction than would otherwise be present. Considered another way,this parallel direction is orthogonal to the stacking direction. Theband modifying layers 50 may also cause the superlattice 25 to have acommon energy band structure, while also advantageously functioning asan insulator between layers or regions vertically above and below thesuperlattice.

Moreover, this superlattice structure may also advantageously act as abarrier to dopant and/or material diffusion between layers verticallyabove and below the superlattice 25. These properties may thusadvantageously allow the superlattice 25 to provide an interface forhigh-K dielectrics which not only reduces diffusion of the high-Kmaterial into the channel region, but which may also advantageouslyreduce unwanted scattering effects and improve device mobility, as willbe appreciated by those skilled in the art.

It is also theorized that semiconductor devices including thesuperlattice 25 may enjoy a higher charge carrier mobility based uponthe lower conductivity effective mass than would otherwise be present.In some embodiments, and as a result of the band engineering achieved bythe present invention, the superlattice 25 may further have asubstantially direct energy bandgap that may be particularlyadvantageous for opto-electronic devices, for example.

The superlattice 25 also illustratively includes a cap layer 52 on anupper layer group 45 n. The cap layer 52 may comprise a plurality ofbase semiconductor monolayers 46. The cap layer 52 may have between 2 to100 monolayers of the base semiconductor, and, more preferably between10 to 50 monolayers.

Each base semiconductor portion 46 a-46 n may comprise a basesemiconductor selected from the group consisting of Group IVsemiconductors, Group III-V semiconductors, and Group II-VIsemiconductors. Of course, the term Group IV semiconductors alsoincludes Group IV-IV semiconductors, as will be appreciated by thoseskilled in the art. More particularly, the base semiconductor maycomprise at least one of silicon and germanium, for example.

Each energy band-modifying layer 50 may comprise a non-semiconductorselected from the group consisting of oxygen, nitrogen, fluorine, carbonand carbon-oxygen, for example. The non-semiconductor is also desirablythermally stable through deposition of a next layer to therebyfacilitate manufacturing. In other embodiments, the non-semiconductormay be another inorganic or organic element or compound that iscompatible with the given semiconductor processing as will beappreciated by those skilled in the art. More particularly, the basesemiconductor may comprise at least one of silicon and germanium, forexample

It should be noted that the term monolayer is meant to include a singleatomic layer and also a single molecular layer. It is also noted thatthe energy band-modifying layer 50 provided by a single monolayer isalso meant to include a monolayer wherein not all of the possible sitesare occupied (i.e., there is less than full or 100% coverage). Forexample, with particular reference to the atomic diagram of FIG. 2, a4/1 repeating structure is illustrated for silicon as the basesemiconductor material, and oxygen as the energy band-modifyingmaterial. Only half of the possible sites for oxygen are occupied in theillustrated example.

In other embodiments and/or with different materials this one-halfoccupation would not necessarily be the case as will be appreciated bythose skilled in the art. Indeed it can be seen even in this schematicdiagram, that individual atoms of oxygen in a given monolayer are notprecisely aligned along a flat plane as will also be appreciated bythose of skill in the art of atomic deposition. By way of example, apreferred occupation range is from about one-eighth to one-half of thepossible oxygen sites being full, although other numbers may be used incertain embodiments.

Silicon and oxygen are currently widely used in conventionalsemiconductor processing, and, hence, manufacturers will be readily ableto use these materials as described herein. Atomic or monolayerdeposition is also now widely used. Accordingly, semiconductor devicesincorporating the superlattice 25 in accordance with the invention maybe readily adopted and implemented, as will be appreciated by thoseskilled in the art.

It is theorized without Applicant wishing to be bound thereto that for asuperlattice, such as the Si/O superlattice, for example, that thenumber of silicon monolayers should desirably be seven or less so thatthe energy band of the superlattice is common or relatively uniformthroughout to achieve the desired advantages. The 4/1 repeatingstructure shown in FIGS. 1 and 2, for Si/O has been modeled to indicatean enhanced mobility for electrons and holes in the X direction. Forexample, the calculated conductivity effective mass for electrons(isotropic for bulk silicon) is 0.26 and for the 4/1 SiO superlattice inthe X direction it is 0.12 resulting in a ratio of 0.46. Similarly, thecalculation for holes yields values of 0.36 for bulk silicon and 0.16for the 4/1 Si/O superlattice resulting in a ratio of 0.44.

While such a directionally preferential feature may be desired incertain semiconductor devices, other devices may benefit from a moreuniform increase in mobility in any direction parallel to the groups oflayers. It may also be beneficial to have an increased mobility for bothelectrons and holes, or just one of these types of charge carriers aswill be appreciated by those skilled in the art.

The lower conductivity effective mass for the 4/1 Si/O embodiment of thesuperlattice 25 may be less than two-thirds the conductivity effectivemass than would otherwise occur, and this applies for both electrons andholes. Of course, the superlattice 25 may further comprise at least onetype of conductivity dopant therein, as will also be appreciated bythose skilled in the art.

Indeed, referring now additionally to FIG. 3, another embodiment of asuperlattice 25′ in accordance with the invention having differentproperties is now described. In this embodiment, a repeating pattern of3/1/5/1 is illustrated. More particularly, the lowest base semiconductorportion 46 a′ has three monolayers, and the second lowest basesemiconductor portion 46 b′ has five monolayers. This pattern repeatsthroughout the superlattice 25′. The energy band-modifying layers 50′may each include a single monolayer. For such a superlattice 25′including Si/O, the enhancement of charge carrier mobility isindependent of orientation in the plane of the layers. Those otherelements of FIG. 3 not specifically mentioned are similar to thosediscussed above with reference to FIG. 1 and need no further discussionherein.

In some device embodiments, all of the base semiconductor portions of asuperlattice may be a same number of monolayers thick. In otherembodiments, at least some of the base semiconductor portions may be adifferent number of monolayers thick. In still other embodiments, all ofthe base semiconductor portions may be a different number of monolayersthick.

In FIGS. 4A-4C, band structures calculated using Density FunctionalTheory (DFT) are presented. It is well known in the art that DFTunderestimates the absolute value of the bandgap. Hence all bands abovethe gap may be shifted by an appropriate “scissors correction.” Howeverthe shape of the band is known to be much more reliable. The verticalenergy axes should be interpreted in this light.

FIG. 4A shows the calculated band structure from the gamma point (G) forboth bulk silicon (represented by continuous lines) and for the 4/1 Si/Osuperlattice 25 shown in FIG. 1 (represented by dotted lines). Thedirections refer to the unit cell of the 4/1 Si/O structure and not tothe conventional unit cell of Si, although the (001) direction in thefigure does correspond to the (001) direction of the conventional unitcell of Si, and, hence, shows the expected location of the Si conductionband minimum. The (100) and (010) directions in the figure correspond tothe (110) and (−110) directions of the conventional Si unit cell. Thoseskilled in the art will appreciate that the bands of Si on the figureare folded to represent them on the appropriate reciprocal latticedirections for the 4/1 Si/O structure.

It can be seen that the conduction band minimum for the 4/1 Si/Ostructure is located at the gamma point in contrast to bulk silicon(Si), whereas the valence band minimum occurs at the edge of theBrillouin zone in the (001) direction which we refer to as the Z point.One may also note the greater curvature of the conduction band minimumfor the 4/1 Si/O structure compared to the curvature of the conductionband minimum for Si owing to the band splitting due to the perturbationintroduced by the additional oxygen layer.

FIG. 4B shows the calculated band structure from the Z point for bothbulk silicon (continuous lines) and for the 4/1 Si/O superlattice 25(dotted lines). This figure illustrates the enhanced curvature of thevalence band in the (100) direction.

FIG. 40 shows the calculated band structure from both the gamma and Zpoint for both bulk silicon (continuous lines) and for the 5/1/3/1 Si/Ostructure of the superlattice 25′ of FIG. 3 (dotted lines). Due to thesymmetry of the 5/1/3/1 Si/O structure, the calculated band structuresin the (100) and (010) directions are equivalent. Thus the conductivityeffective mass and mobility are expected to be isotropic in the planeparallel to the layers, i.e. perpendicular to the (001) stackingdirection. Note that in the 5/1/3/1 Si/O example the conduction bandminimum and the valence band maximum are both at or close to the Zpoint.

Although increased curvature is an indication of reduced effective mass,the appropriate comparison and discrimination may be made via theconductivity reciprocal effective mass tensor calculation. This leadsApplicant to further theorize that the 5/1/3/1 superlattice 25′ shouldbe substantially direct bandgap. As will be understood by those skilledin the art, the appropriate matrix element for optical transition isanother indicator of the distinction between direct and indirect bandgapbehavior.

Having now described the structure and formation of example MSTmaterials, various embodiments of semiconductor devices and method fortheir manufacture will now be described which advantageously provide formetal-silicon contacts with proximate dopants using the above-describedMST materials. By way of background, it is typical in semiconductordevices for electrons to be transferred between a semiconductor (such assilicon) and conducting metal “interconnects” which transfer chargebetween semiconductor devices. Electrical resistance between thesemiconductor and metal increases the energy required and reduces themaximum speed of computations and other functions performed by circuitsusing the devices. It is thus advantageous to minimize this electricalresistance.

When electrons are transferred between a metal and a semiconductor suchas silicon, there is a potential barrier encountered by the electron.This barrier is typically referred to as the “Schottky barrier.”Electrons can either have sufficient kinetic energy to overcome theSchottky barrier directly, or electrons with lower kinetic energy maypass between the metal and semiconductor via quantum mechanicaltunneling. Such tunneling is more likely the spatially thinner theSchottky barrier. A typical way to attain a reduced barrier is toincrease the electric field. Higher levels of ionized impurities(“dopants”) typically yield higher electric fields and thus increase thetunneling probability, increasing the electron flux between the metaland the semiconductor, thus reducing the effective electricalresistance. But in addition to increasing the electric field, highlevels of impurities may additionally reduce the Schottky barrieritself, by reducing the effective band gap of the semiconductorimmediately adjacent to the metal-semiconductor interface (and via otherchemical effects). This effect is evident from density functional theorycalculations.

Oxygen insertion (OI) layers (or other non-semiconductor layers), suchas provided in an MST film, for example, may contribute to a lowerSchottky and/or thinner barrier by trapping dopants. Density functionaltheory calculations have shown that OI layers provide for the favorablesubstitution of specific dopant atoms for silicon atoms within one ortwo atomic layers of the OI layer. By trapping dopants immediatelyproximate to, for example, separated by one or two atomic layers from, ametal-semiconductor interface, OI layers may thus contribute to arelatively higher concentration of dopants immediately adjacent to themetal-semiconductor interface, increasing the electric field, andadditionally reducing the Schottky barrier.

In addition to trapping dopants, OI layers may immobilize them bytrapping point defects that would otherwise aid the diffusion ofdopants. So, while an OI layer proximate to a metal-semiconductor layermay trap dopants immediately adjacent to that interface, additional OIlayers further from but still proximate to the interface may trap pointdefects that would otherwise aid in the diffusion of dopants away fromthe interface. Thus, it may be beneficial to have more than one, forexample two, three, or four, OI layers proximate to the interface.

Generally speaking, the embodiments described herein utilize one or moreoxygen insertion (“OI” or “MST”) layers in close proximity to ametal-semiconductor interface in conjunction with a high (for example,in excess of 10²¹/cm³, or 2% of crystalline sites in the siliconlattice) of ionized impurities such as boron, phosphorus, arsenic,antimony, indium, or gallium in the near proximity (for example, closerthan 1 nm) to the metal-silicon interface. Examples of metals includealuminum, tungsten, nickel, titanium, copper, cobalt, indium, gold,platinum, erbium, ytterbium, and compounds of any of these metals withsilicon or germanium.

Since the OI layers may provide for favorable substitution by dopantatoms of silicon atoms one or two atomic layers away, the most favorableseparation of an OI layer from a metal-semiconductor interface is one ortwo atomic layers, allowing for the trapping of a high concentration ofdopants up to and immediately adjacent to the metal-semiconductorinterface. But benefits may also be provided with other separations, forexample three, or four atomic layers. Additionally, there may be abenefit in including additional oxygen insertion layers in addition tothis first oxygen insertion layer.

Examples of this approach are represented in the graph 500 of FIG. 15,in which: Si=Silicon atom; M=Metal atom (e.g. titanium); O=Oxygen atom;and D=Dopant atom (e.g. boron). The oxygen atoms as drawn are part ofoxygen insertion layers, where the oxygen is bonded with adjacentsilicon atoms. While oxygen atoms are necessarily present in the OIlayers, there may also additionally be nitrogen atoms, which are notrepresented in the figure. The presence of nitrogen may be beneficialfor the trapping of dopants or for the thermal stability of the OIlayers, for example.

Similarly, the represented dopants are replacing silicon atoms in thecrystalline lattice, as opposed to occupying “interstitial” positions orin dopant clusters where they will fail to contribute to free carriersin the semiconductor. However, while substitutional dopants arerepresented in the figure, a high concentration of dopants trapped nearthe metal-semiconductor interface may reduce a Schottky barrier withalternate atomic configurations. The positions of dopants in the diagramare a schematic representative for illustrational purposes. An actualdistribution of dopant atoms will be in part random, influenced by thespecific atomic configuration of oxygen atoms, and the local bonding ofatoms. The illustrated configurations represent the distance of atomsfrom the metal-semiconductor interface, not specific positions of atomswithin layers. The illustrated configurations are as follows:

-   -   (a) OI layer in contact with metal, trapping dopants below the        OI layer;    -   (b) OI layer separated by one atomic layer of silicon from the        metal, trapping dopants both above and below the OI layer;    -   (c) OI layer separated by two atomic layers from the metal,        trapping dopants both above and below the OI layer;    -   (d) OI layer separated by three atomic layers from the metal,        trapping dopants both above and below the OI layer, but in this        example not reaching the metal interface itself;    -   (e) OI layer separated by four atomic layers from the metal,        trapping dopants both above and below the OI layer, but in this        example not reaching the metal interface itself.

In addition to these configurations, additional configurations withmultiple OI layers are also possible, for example one layer separatedfrom the metal-semiconductor interface by two silicon layers, and anadditional layer separated by an additional four atomic layers. Thesemultiple layers may provide for dopant trapping both at themetal-semiconductor interface, and additionally below themetal-semiconductor interface, the former contributing to chemicalSchottky barrier lowering and a higher electric field, the lattercontributing primarily through higher electric field. The embodimentsset forth herein are generally defined by the presence of a layerproximate to the metal-semiconductor interface in conjunction with ahigh concentration of dopants, but does not exclude additional layers ordopant atoms not proximate to OI layer(s). A specific advantage ofadditional OI layers is these additional layers may increase thestability of the structure, for example blocking the loss of oxygen fromthe layer closest to the metal-semiconductor interface, or trappingpoint defects which otherwise would contribute to a loss of dopant atomsfrom the region proximate to the metal-semiconductor interface.

Referring now to FIG. 5, the above-described superlattice structures mayadvantageously be used in semiconductor devices to provide reducedsource/drain contact resistance by applying the above-describedprinciples. In typical semiconductor processes, the reduction of metalto semiconductor contact area requires lower contact resistivity (e.g.,p_(c)<1E-8 ohm·cm²). Contact resistivity is determined by twoparameters, which are:

-   -   N_(D): active dopant concentration at metal/semiconductor        interface; and    -   ΦF_(Bo): Schottky barrier height at metal/semiconductor        interface        Furthermore, the metal-semiconductor Schottky barrier height is        “pinned” for different metals. Moreover, an interfacial        insulator can “de-pin” the Fermi level depending on thickness,        bandgap, and permittivity.

In the semiconductor device 100 shown in FIG. 5 (a FET), a dopantdiffusion blocking superlattice 125 (such as those described above inFIGS. 1-4C) is used to advantageously increase surface dopantconcentration to allow a higher N_(D) during in-situ doped epitaxialprocessing by preventing diffusion into a channel region 130 of thedevice. More particularly, the device 100 illustratively includes asemiconductor layer or substrate 101, and spaced apart source and drainregions 102, 103 formed in the semiconductor layer with the channelregion 130 extending therebetween. The dopant diffusion blockingsuperlattice 125 illustratively extends through the source region 102 todivide the source region into a lower source region 104 and an uppersource region 105, and also extends through the drain region 103 todivide the drain region into a lower drain region 106 and an upper drainregion 107.

The dopant diffusion blocking superlattice 125 may also conceptually beconsidered as a source dopant blocking superlattice within the sourceregion 102, a drain dopant blocking superlattice within the drain region103, and a body dopant blocking superlattice beneath the channel 130,although in this configuration all three of these are provided by asingle blanket deposition of the MST material across the substrate 101as a continuous film. The semiconductor material above the dopantblocking superlattice 125 in which the upper source/drain regions 105,107 and channel region 130 are defined may be epitaxially grown on thedopant blocking superlattice 125 either as a thick superlattice caplayer or bulk semiconductor layer, as discussed further above. In theillustrated example, the upper source/drain regions 105, 107 may each belevel with an upper surface of this semiconductor layer (i.e., they areimplanted within this layer).

As such, the upper source/drain regions 105, 107 may advantageously havea same conductivity as the lower source/drain regions 104, 106, yet witha higher dopant concentration. In the illustrated example, the uppersource/drain regions 105, 107 and the lower source/drain regions 104,106 are N-type for a N-channel device, but these regions may also beP-type for an P-channel device as well (this applies to otherconfigurations described herein as well). Surface dopant may beintroduced by ion implantation, for example. Yet, the dopant diffusionis reduced by the MST film material of the diffusion blockingsuperlattice 125 because it traps point defects/interstitials introducedby ion implantation which mediate dopant diffusion.

The semiconductor device 100 further illustratively includes a gate 108on the channel region 130. The gate illustratively includes a gateinsulating layer 109 gate electrode 110. Sidewall spacers 111 are alsoprovided in the illustrated example.

Referring now to FIG. 6, in accordance with another exampleimplementation a semiconductor device 200 (FET) illustratively includesa semiconductor layer or substrate 201, and spaced apart source anddrain regions 202, 203 with a channel region 230 extending therebetween.In the illustrated embodiment, a source diffusion blocking superlattice225 s illustratively extends through the source region 202 to divide thesource region into a lower source region 204 and an upper source region205. Similarly, a drain diffusion blocking superlattice 225 d extendsthrough the drain region 203 to divide the drain region into a lowerdrain region 206 and an upper drain region 207. Consideredalternatively, the upper source and drain regions 205, 207 are eachraised above an upper surface of the semiconductor layer 201, and thereis no superlattice extending between the source and drain regions 202,203 as in the example of FIG. 5 (i.e., beneath gate 208). The gate 208illustratively includes a gate insulator 209 and gate electrode 210, andgate sidewall spacers 211 may also be provided.

In this implementation, the surface dopant in the upper source/drainregions 205, 207 may be introduced by selectively growing MST films,followed by in-situ doped epi film formation. Here again, the MSTmaterial of the source/drain dopant diffusion blocking superlattices 225s, 225 b advantageously helps prevent dopant diffusion into the channelregion 230, and thus allows higher surface dopant concentration as notedabove.

In accordance with another example implementation now described withreference to FIGS. 7A-7C, further processing steps may be performed tothe semiconductor device 100 shown in FIG. 5 to perform Schottky barrierheight modulation by controlling a thickness and composition of aninterfacial insulator. With traditional metal contacts, too thick of asource/drain insulator results in high contact resistivity due to hightunneling resistance. However, the superlattice 125′ advantageouslyprovides desired non-semiconductor (e.g., oxygen) dose control forFermi-level de-pinning and tunneling resistance. After formation of thegate 108′ on the channel region 130′, co-implantation of N, C, or F mayfurther modulate insulator composition for lower permittivity (e.g., Cand F used for low-k ILD film to modulate SiO₂ composition), asillustrated in FIG. 7A. The MST film of the superlattice 125′effectively accumulates these elements into the surface region. Itshould be noted that in some embodiments N and C may be incorporatedinto silicon surface by gaseous form (e.g., N₂ anneal or CO, CH₄ anneal)instead of co-implantation.

A thermal treatment and metal deposition may then be performed (FIGS.7B-7C). The thermal treatment moves upward non-semiconductor atoms(oxygen in the present example) from the non-semiconductor monolayers ofthe dopant diffusion blocking superlattices 125′ which react with themetal to form respective source and drain contact insulating interfaces140′, 141′ between the upper source and drain regions and adjacentportions of metal layers 142′, 143′ formed by the metal deposition.Stated alternatively, as the oxygen atoms disassociate from thesuperlattice 125′ in the source and drain regions and move upward toform the contact insulating interfaces 140′, 141′ so that there is nolonger a defined superlattice layer separating the lower/upper sourceregions 104′, 105′ and lower/upper drain regions 106′, 107′ (see FIG.7B).

In accordance with one example implementation, a Co/Co_(0.75)Ti_(0.25)(2 nm) metal deposition may be performed at a temperature in a range ofabout +200˜400 C for approximately 10 min. to form the source and draincontact insulating interfaces 140′, 141′ and metal layers 142′, 143′.Moreover, in some implementations, an additional metal deposition (e.g.,Co) may be performed to form upper source/drain metal contact layers144′, 145′ in the semiconductor device 100′.

Another example embodiment similar to the semiconductor device 200 isnow described with reference to FIG. 8. In this illustrated example, thesource and drain dopant diffusion blocking superlattices 225 s′, 225 d′advantageously provide for Schottky barrier height modulation viahetero-epitaxial film integration. More particularly, the lower sourceand drain regions 204′, 206′ include a different material than the uppersource and drain regions 205′, 207′. In this example, the lower sourceand drain regions 204′, 206′ are silicon, and the upper source and drainregions 205′, 207′ are SiGeC, although different materials may be usedin different embodiments.

Because the MST material is effective in integrating hetero-epitaxialsemiconductor material, incorporation of C(1-2%) to Si or SiGe on Si mayinduce a positive conduction band offset. More particularly, this is aSiGeC/MST/n+Si structure that is effective for reducing Schottky barrierheight.

Referring additionally to FIG. 9, another similar semiconductor device200″ advantageously provides for Schottky barrier height modulation viahetero-epitaxial film integration. In the illustrated example, thesemiconductor layer/substrate 201″ is silicon germanium, and the lowersource/drain regions 204″, 206″ are P+ SiGe. Moreover, the uppersource/drain regions 205″, 207″ are also silicon, and a respectiveplatinum contact layer 242″, 243″ is formed on each of the uppersource/drain regions. The upper source/drain regions 205″, 207″ may beformed in a relatively thin epitaxial silicon layer (e.g., 2-5 nm). Hereagain, the MST material is effective in integrating hetero-epitaxialsemiconductor material, and strained Si on SiGe (or Ge) mayadvantageously induce negative valence band offset. As a result, theillustrated s-Si/MST/p+SiGe structure may be effective for reducingSchottky barrier height as well.

Turning now additionally to FIG. 10, certain of the above-describedplanar FET configurations may also advantageously be implemented invertical semiconductor devices as well. An example FINFET 300illustratively includes a semiconductor layer or substrate 301 (e.g.,silicon), an insulating layer 350 on the substrate (e.g., SiO₂), and oneor more semiconductor fins 351 extending vertically upward from thesubstrate through the insulating layer.

Spaced apart source and drain regions 302, 303 are formed in eachsemiconductor fin 351 with a channel region 330 extending therebetween.A source dopant diffusion blocking superlattice 325 s extends throughthe source region 302 to divide the source region into a lower sourceregion 304 and an upper source region 305, and a drain dopant diffusionblocking superlattice 325 d extends through the drain region 303 todivide the drain region into a lower drain region 306 and an upper drainregion 307. Similar to the embodiment of FIG. 6, the upper source/drainregions 305, 307 have a same conductivity and higher dopantconcentration (N++) than the lower source/drain regions 304, 306 (N+).Moreover, the upper source/drain regions 305, 307 extend above an uppersurface of the semiconductor fin 351. That is, superlattices 325 s, 325d may be formed on the top of the semiconductor fin 351, and the uppersource/drain regions 305, 307 may be epitaxially grown on the respectivesuperlattices. In this regard, the upper source/drain regions 305, 307may implanted with a dopant, or they may be in-situ doped epitaxiallayers as described above.

The FINFET 300 also illustratively includes a gate 308 overlying thechannel regions 330 of the fins 351. The gate 308 illustrativelyincludes a gate insulator 309 and a gate electrode 310 on the gateinsulator.

In accordance with another example implementation, a FINFET 300′ may befabricated using a similar process to that described above with respectto FIGS. 7A-7C, i.e., involving a thermal treatment and metal depositionto define source/drain insulating layers 340′, 341′ between the uppersource/drain regions 305′, 307′ and metal layers 342′, 343′ (e.g.,CoTi_(x)) respectively. Upper metal layers 344′, 345′ (e.g., Co) mayalso be formed on the lower metal layers 342′, 343′ in some embodiments.Here again, the heat treatment causes the non-semiconductor atoms in thesource/drain dopant blocking superlattice layers to move upward, leavingno superlattice between the upper source/drain regions 305′, 307′ andthe lower source/drain regions 304′, 306′ in the final FINFET 300′. Hereagain, this approach advantageously provides for Schottky barrier heightmodulation by controlling the thickness and composition of theinterfacial insulators 340′, 341′.

Referring additionally to FIG. 13, a FINFET 300″ is provided which issimilar to embodiment of FIG. 8 above, in that the source and draindopant blocking superlattices 325 s″, 325 d″ advantageously provide forSchottky barrier height modulation via hetero-epitaxial filmintegration. More particularly, the lower source and drain regions 304″,306″ include a different material than the upper source and drainregions 305″, 307″. In this example, the lower source and drain 304″,306″ regions are silicon, and the upper source and drain regions 305″,307″ are SiGeC, although different materials may be used in differentembodiments. Also, the lower metal layers 342″, 342″ are titanium, andthe upper metal layers 344″, 345″ are cobalt in the illustrated example.

In still another example shown in FIG. 14, similar to the embodiment ofFIG. 9 the FINFET 300′″ may also provide for Schottky barrier heightmodulation via hetero-epitaxial film integration. In the illustratedexample, the semiconductor fin 351′ is silicon germanium, and the lowersource/drain regions 304″, 306″ are P+SiGe. Moreover, the uppersource/drain regions 305″, 307″ are silicon, and a respective platinumcontact layer 342″, 343″ is formed on each of the upper source/drainregions. Here again, the upper source/drain regions 305″, 307″ may beformed in a relatively thin epitaxial silicon layer (e.g., 2-5 nm).

In the above-described examples, a dopant blocking superlattice is shownin both of the source and drain regions of the illustrated devices.However, it should be noted that in some embodiments both of the sourceand drain regions need not have a dopant blocking superlattice. That is,the dopant blocking superlattice may be in just one of the source ordrain regions in some embodiments.

Furthermore, turning now to FIG. 16, in some embodiments a dopantblocking superlattice may also be incorporated in a body contact toprovide reduced body contact resistance in addition to, or instead of,in the source/drain regions. In the illustrated example, a semiconductordevice 400 (here a planar FET) includes a semiconductor layer orsubstrate 401, spaced apart source and drain regions 402, 403 (whichhave respective lightly doped source/drain extensions 404, 405 in thisexample) in the semiconductor layer 401 with a channel region 430extending therebetween, and a gate 408 on the channel region withsidewall spacers 411. As similarly described above, the gate 408illustratively includes a gate insulator 409 and a gate electrode 410.The semiconductor device 400 further illustratively includes a bodycontact 420 in the semiconductor layer 401 and comprising a body contactdopant diffusion blocking superlattice 425 extending through the bodycontact to divide the body contact into a first body contact region 421and a second body contact region 422. As similarly described above, thesecond body contact region 422 has a same conductivity and higher dopantconcentration than the first body contact region 421. Here again, thebody contact dopant diffusion blocking superlattice 425 may be similarto those superlattice structures described above, and the materials usedfor, and dopant concentrations within, the first and second body contactregions 421, 422 may also be similar to those described above to providedesired contact resistance reduction.

In another example device 400′ shown in FIG. 17, a similar configurationis provided which has a back-side body contact 420′ instead of thetop-side or front-side body contact 420 shown in FIG. 16. The remainingcomponents are similar to those discussed with reference to FIG. 16 andaccordingly require no further discussion herein. It should be notedthat in the back-side implementation the first and second regions 421′,422′ are flipped vertically with respect to the first and second regions421, 422 in the device 400, as they are on the opposite side of thedevice 400′.

Turning now to FIG. 18, another example contact structure 500 is nowdescribed which may be also be used in certain implementations ofsource/drain, body, or other contacts to provide reduced Schottkybarrier height and thereby decreased contact resistance. The contact 500is formed in a semiconductor layer 501. The contact illustrativelyincludes one or more oxygen monolayers 550 constrained within a crystallattice of adjacent semiconductor portions 546 a, 546 b of thesemiconductor layer 501. The oxygen monolayer(s) 550 is and spaced apartfrom a surface of the semiconductor layer 501 by between one and fourmonolayers (a four monolayer spacing is shown in the semiconductorportion 546 b in the illustrated example). Furthermore, a metal layer531 (which may include the same metals discussed above) is formed on thesurface of the semiconductor layer 501 above the oxygen monolayer(s)550. The contact 500 may be formed on a semiconductor layer or substrate521.

By way of example, a dopant concentration within the portion 546 b(i.e., between the oxygen monolayer(s) 550 and the metal layer 531) maybe 1×10²¹ atoms/cm³ or greater (although lower concentrations may alsobe used in different embodiments). Considered alternatively, a dopantconcentration equivalent to approximately 2% of the crystalline sites ina silicon lattice may advantageously be occupied by dopant atoms in anexample configuration. This is based upon an estimate for the maximumdistance range over which the oxygen monolayer can directly trap dopants(e.g., boron) sufficiently close to the metal to directly reduce theSchottky barrier of the contact, as opposed to reducing diffusion, yetwhile otherwise retaining a conventional doped metal-semiconductorinterface with a bulk-line environment for the dopant atoms, and theminimum concentration of dopants where a benefit will be realized.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

That which is claimed is:
 1. A FINFET comprising: a semiconductor fin;spaced apart source and drain regions in the semiconductor fin with achannel region extending therebetween; at least one dopant diffusionblocking superlattice dividing at least one of the source and drainregions into a lower region and an upper region with the upper regionhaving a same conductivity and higher dopant concentration than thelower region; the dopant diffusion blocking superlattice comprising aplurality of stacked groups of layers, each group of layers comprising aplurality of stacked base semiconductor monolayers defining a basesemiconductor portion, and at least one non-semiconductor monolayerconstrained within a crystal lattice of adjacent base semiconductorportions; and a gate on the channel region.
 2. The FINFET of claim 1wherein the at least one dopant diffusion blocking superlatticecomprises a respective dopant diffusion blocking superlattice for eachof the source and drain regions.
 3. The FINFET of claim 1 wherein theupper region is raised above an upper surface of the semiconductorlayer.
 4. The FINFET of claim 1 wherein the lower region comprises adifferent material than the upper region.
 5. The FINFET of claim 1further comprising a metal contact on the upper region.
 6. The FINFET ofclaim 1 further comprising a semiconductor substrate from which thesemiconductor fin extends vertically upward, and an insulator on thesemiconductor substrate and on opposite sides of the semiconductor fin.7. The FINFET of claim 1 wherein the base semiconductor monolayerscomprise silicon.
 8. The FINFET of claim 1 wherein the at least onenon-semiconductor monolayer comprises oxygen.
 9. The FINFET of claim 4wherein the lower region comprises silicon; and wherein the upper regioncomprises silicon germanium.
 10. The FINFET of claim 4 wherein the lowerregion comprises silicon germanium; and wherein the upper regioncomprises silicon.
 11. The FINFET of claim 5 wherein the metal contactcomprises at least one of titanium, cobalt, nickel and platinum.
 12. AFINFET comprising: a semiconductor fin; spaced apart source and drainregions in the semiconductor fin with a channel region extendingtherebetween; a source dopant diffusion blocking superlattice extendingthrough the source region to divide the source region into a lowersource region and an upper source region with the upper source regionhaving a same conductivity and higher dopant concentration than thelower source region; a drain dopant diffusion blocking superlatticeextending through the drain region to divide the drain region into alower drain region and an upper drain region with the upper drain regionhaving a same conductivity and higher dopant concentration than thelower drain region; the upper drain region and upper source region eachbeing raised above an upper surface of the semiconductor layer; each ofthe source dopant diffusion blocking superlattice and drain dopantdiffusion blocking superlattice comprising a respective plurality ofstacked groups of layers, each group of layers comprising a plurality ofstacked base semiconductor monolayers defining a base semiconductorportion, and at least one non-semiconductor monolayer constrained withina crystal lattice of adjacent base semiconductor portions; and a gate onthe channel region.
 13. The FINFET of claim 12 wherein the lower drainregion comprises a different material than the upper drain region; andthe lower source region comprises a different material than the uppersource region.
 14. The FINFET of claim 12 further comprising a metalsource contact on the upper source region, and a metal drain contact onthe upper drain region.
 15. The FINFET of claim 12 further comprising asemiconductor substrate from which the semiconductor fin extendsvertically upward, and an insulator on the semiconductor substrate andon opposite sides of the semiconductor fin.
 16. The FINFET of claim 13wherein the lower source and drain regions comprise silicon; and whereinthe upper source and drain regions comprise silicon germanium.
 17. TheFINFET of claim 13 wherein the lower source and drain regions comprisesilicon germanium; and wherein the upper source and drain regionscomprise silicon.
 18. The FINFET of claim 14 wherein the metal sourceand drain contacts comprise at least one of titanium, cobalt, nickel andplatinum.
 19. A FINFET comprising: a semiconductor fin; spaced apartsource and drain regions in the semiconductor fin with a channel regionextending therebetween; a source dopant diffusion blocking superlatticeextending through the source region to divide the source region into alower source region and an upper source region with the upper sourceregion having a same conductivity and higher dopant concentration thanthe lower source region; a drain dopant diffusion blocking superlatticeextending through the drain region to divide the drain region into alower drain region and an upper drain region with the upper drain regionhaving a same conductivity and higher dopant concentration than thelower drain region; each of the source dopant diffusion blockingsuperlattice and drain dopant diffusion blocking superlattice comprisinga respective plurality of stacked groups of layers, each group of layerscomprising a plurality of stacked base semiconductor monolayers defininga base semiconductor portion, and at least one non-semiconductormonolayer constrained within a crystal lattice of adjacent basesemiconductor portions; a gate on the channel region; and a metal sourcecontact on the upper source region, and a metal drain contact on theupper drain region; wherein the lower drain region comprises a differentmaterial than the upper drain region, and the lower source regioncomprises a different material than the upper source region.
 20. TheFINFET of claim 19 wherein the lower source and drain regions comprisesilicon; and wherein the upper source and drain regions comprise silicongermanium.
 21. The FINFET of claim 19 wherein the lower source and drainregions comprise silicon germanium; and wherein the upper source anddrain regions comprise silicon.
 22. The FINFET of claim 19 wherein themetal source and drain contacts comprise at least one of titanium,cobalt, nickel and platinum.